We have further characterized the avian retrovirus reverse transcriptase: 1) We have reported the in vitro isolation of chymotryptic fragments of avian myeloblastosis virus alpha beta DNA polymerase similar to the in vivo isolated pp32 protein in size, sequence and DNA endonuclease activity. 2) We have finally accomplished the partial NH2-terminal amino acid sequence analysis of the avian myeloblastosis virus reverse transcriptase subunits. This very important observation suggests that the mRNA for gag-pol is a spliced mRNA. 3. The purification of the avian retrovirus reverse transcriptase has been improved by development of heparin-Sepharose chromatography. 4) Finally, we have demonstrated that the avian retrovirus pp32 DNA endonuclease is partially phosphorylated in vivo.